In the railway rolling stock art, it is common practice to support the opposed ends of a freight railcar body on spaced-apart wheel-truck assemblies for travel along a railway track. A standard railcar wheel-truck assembly generally has a laterally spaced pair of sideframes which are, longitudinally operable along the tracks and parallel to the longitudinal axis of the railcar. A bolster, which is transversely positioned to the longitudinal direction of the railcar, couples the sideframes and has the freight car body supported on bolster center plate sections. A railcar wheel-truck, or truck is positioned at the opposed ends of the railcar to support it during its traversal of the rail tracks.
Each sideframe includes a window portion for the bolster ends and the spring groups supporting the bolster, which structure allows bolster movement relative to the sideframe. Each spring group typically includes a plurality of coil springs extending between a sideframe spring seat portion and an undersurface of the bolster end spaced above the respective sideframe spring-seat. Elastomeric spring type products may also be utilized in a spring group as an alternative to the coil springs.
Railway track conditions can include rail running surface variations or discontinuities from differential settling of track on its ballast, rail wear, corrugations, rail misalignment, worn switch frogs or misaligned switch points, as well as the intersection of rails for flange clearance, switches where switching points match with running rails, and rail joints. During normal railcar usage or operation, these and other variations can result in wheel-truck oscillations, which may induce the railcar body to bounce, sway, rock or engage in other unacceptable motions. Wheel-truck movements transferred through the suspension system may reinforce and amplify the uncontrolled motions of the railcar from track variations, which action may result in wheel-truck unloading, and a wheel or wheels of the truck may lift from the track.
The American Association of Railroads, the AAR, establishes the criteria for railcar stability, wheel loading and spring group structure, which criteria are very severe. These criteria are set or defined in recognition that railcar body dynamic modes of vibration, such as rocking of sufficient magnitude, may compress individual springs of the spring group at alternate ends of the bolster, even to a solid or near-solid condition. This alternate-end spring compression is followed by an expansion of the springs, which action-reaction can amplify and exaggerate the `apparent` wheel loading on the suspension system and subsequent rocking motion of the railcar, as opposed to the actual or "average" weight or load from the railcar and freight therein. As a consequence of the amplified rocking motion, and at large amplitudes of such rocking motion, the contact force between the rails and the wheels can be dramatically reduced on the alternate lateral sides of the railcar. In an extreme case, the wheels can elevate and disalign from the track, which enhances the opportunity for a derailment. At a no-load condition of a railcar, one standard considers or uses the rail wheel just contacting the rail as a reference condition.
There are various modes of motion of a railcar body, that is bounce, pitch, yaw, and lateral oscillation, as well as the above-noted roll. In car body roll, or twist and roll as defined by the AAR, the car body appears to be alternately rotating in the direction of either lateral side and about a longitudinal axis of the railcar. Car body pitch can be considered a forward to rearward rotational motion about a transverse railcar axis of rotation, such that the railcar may appear to be lunging between its forward and reverse longitudinal directions. The above-noted car body bounce refers to a vertical and linear motion of the railcar. Yaw is considered a rotational motion about a vertical axis extending through the railcar, which gives the appearance of the car ends moving to and fro as the railcar moves down a track. Finally, lateral stability is considered an oscillating lateral translation of the car body. Alternatively, truck hunting refers to a parallelogramming or warping of the railcar truck, not the railcar body, which is a separate phenomena distinct from the railcar body motions noted above. All of these motion modes are undesirable and can lead to unacceptable railcar performance, as well as contributing to unsafe operation of the railcar.
A common apparatus utilized to control the dynamic responses of railcar trucks and bodies is a friction shoe assembly, which provides bolster-to-sideframe damping of oscillating motion. Friction shoes include a friction wedge in a bolster pocket, which wedge is biased to maintain frictional engagement with the sideframe. Friction shoes dissipate suspension system energy by frictionally damping relative motion between the bolster and sideframe. Various types of friction shoes have been utilized in railway freight car trucks for over forty years.
The structure, the profiles and related elements of the friction shoe have changed since their introduction, which changes included the implementation of elastomeric friction elements on bearing faces or wings of the friction shoes. Winged friction shoes are most generally utilized with constant or fixed bias frictional damping structures with the friction shoe wings contacting complementary inner surfaces of the bolster pockets. A retention or control spring, which biases the friction shoe and maintains it against the bolster pocket surface and the sideframe column wear surface, is supported by the spring base or seat portion of the sideframe beneath the friction shoe. With a fixed or constant bias or damping spring group the control springs do not carry load and, the compression rate of the friction shoe assembly spring, that is the spring displacement as a function of the force, remains essentially unchanged during relative movement between the bolster and sideframe. Thus, in a constant bias arrangement, the biasing force applied to the friction shoe remains constant throughout the operating ranges for both the relative motion and biasing spring displacement between the bolster and sideframes for all conditions of railcar loading. Consequently, the frictional force between the friction shoe and column wear surfaces remains relatively constant.
Alternatively, the frictional response of friction shoes in variable bias arrangements varies with variations in the compression distance of the retention spring. Therefore, the frictional force between the friction shoe and the sideframe column varies with the vertical movement of the bolster. However, in a variable rate spring structure the operating range, or the spring rate, of the control spring may not be adequate to respond to the applied forces, that is the railcar weight and the oscillating dynamic forces, from variations in the track and operating conditions. In at least some variable friction force arrangements, the distance between the friction shoe and the sideframe spring seat has been considered to be adequate to accommodate a friction-shoe biasing spring with a suitable design characteristic to handle the force variations and ranges in the railcar wheel-truck assembly, even for railcars with a higher-rated, load-bearing capacity.
In fixed or constant biasing arrangements, the friction shoe frequently has a spring pocket to receive a control spring having adequate length and coil diameter to provide the requisite frictional damping. In some literature, it has been noted that this damping is particularly designed to accommodate the empty or lightly loaded car condition, but its response at the fully-laden car condition is not considered adequate. Alternatively, a constant-bias spring group adequate for operation at the fully-laden car condition may not provide the requisite damping and suspension response at the empty-car condition.
Illustrative of a problem encountered in railcar-track interactions is the superelevated curve, which is a relatively steeply sloped track section, that is similar to the steeply sloped curves of racetracks or sharply curved roads, to promote high speed travel through a curve. As a railcar enters a superelevated curve, the inside entry wheel will lift or "unload" relative to the outside wheel, although it will not necessarily lift off the track. However, this same wheel will `drop` back down to the rail and the reference plane of the track, relative to the outside wheel, as the railcar leaves the superelevated curve. There are apparent conflicts and contradictions among the desired operating characteristics for railcars which depend upon the state of the railcar weight between empty and fully laden. The variation in the railcar weights impacts upon the desired spring rates. Further, these railcar variations have been compounded by the expanding differential between the decreasing tare weight of the empty railcar and the increasing rated-carrying-capacity of the newer-designed railcars.
The spring group arrangements support the railcar and damp the relative interaction between the bolster and sideframe. There have been numerous types of spring groups utilized for railcar suspension systems, such as concentric springs within the spring group; five, seven and nine spring arrangements; elongated springs for the friction shoe; and, short spring-long spring combinations for the friction shoe within the multispring set. These are just a few of the many noted spring arrangements that have been positioned between sideframe and bolster end assemblies. These spring assemblies must conform to standards set by the Association of American Railroads (AAR), which prescribes a fixed spring height for each coil spring at the fully-compressed or solid spring condition. The particular spring arrangement for any railcar is dependent upon the physical structure of the railcar, its rated weight-carrying capacity and the structure of the wheel-truck assembly. That is, the spring group arrangement must be responsive to variations in the railcar such as the empty railcar weight, the laden-to-capacity railcar weight, railcar weight distribution, railcar operating characteristics, available vertical space between the sideframe spring-platform and the bolster end, the specific friction shoe design and, other operating and physical parameters.
The following table illustrates the change in 100-ton railcar empty weights and lading capacities for several periods to for both the gross rail load and the railcar weight, which car and lading capacity weights for some periods are overlapping:
__________________________________________________________________________ TIME PERIOD (years) TARE WEIGHT OF LIGHT WEIGHT CARS (approximate) (pounds) __________________________________________________________________________ 1950's 68,000 1970's 63,000 1980's 52,000 1990's 44,000 design goal 40,000 __________________________________________________________________________ TIME PERIOD GROSS RAIL LOAD (pounds) __________________________________________________________________________ 1950 TO 1975 242,000 1975 to 1990's 263,000 1990's 286,000 under review 297,000 __________________________________________________________________________
The continuing divergence between the empty-railcar weight and the freight-load carrying capacity of the railcars produces changes in the railcar operating characteristics. That is, as the railcars become lighter to reduce the non-tariff railcar load or weight, there has been a continuing demand to increase the carrying capacity of the railcar to further enhance revenue-generation from each railcar. This has resulted in an increase in the difference between the tare weight and the capacity weight of the railcar. The operating characteristics of the railcar can be accommodated at either the empty-car weight or fully-loaded car weight extreme with only nominal adjustment of the suspension systems from the railcar designer. However, as the differential between the car weight and the car-lading capacity increases, it becomes progressively more difficult to provide a spring group and damping apparatus, which will fit the physical structural limitations of a railcar and be operable over a wide range of loads and broad variations in the railcar operating environment, such as rail track variations.
Currently many 100-ton railcars, and in fact, most new, coal railcars with a gross load capacity of 286,000 pounds and a tare weight of 43,000 pounds, are in service, and the rail industry has an ultimate goal of providing a railcar tare weight of less than 40,000 pounds with at least this rated carrying capacity. The railcar must be physically able to bear the rated load weight and maintain contact with the track as the car travels at varying speeds along different track contours with varying track conditions. Simultaneously, the railcar and truck assemblies must have operating characteristics enabling it to be safely operable on these same varying track conditions at the unloaded, empty-car condition. Both operating weight extremes must be accommodated without posing the danger of imminent derailment for either condition.
To provide a railcar with the above-required operating range capabilities, the damping system spring group incorporated into the truck assembly must have both static and dynamic operating characteristics to accommodate these wide laden-to-empty weight ranges. That is, operation of a large, empty-volume freight car in motion on a railtrack with a wide variant of track and contour conditions can lead to dynamic operating problems from oscillations, which can progress to uncontrolled instabilities of the railcar especially in superelevated curves. Track-to-wheel separation is a result of several conditions, including traversal of rail imperfections, and in conjunction with the oscillation frequency of the car from traversing the disuniform tracks, disengagement of a wheel of an unloaded railcar is not an unusual condition. Although wheel disengagement from the track does not generally result in a derailment, the implied hazard from such a separation is readily apparent and should be avoided, if possible.
Freight railcars are continuously becoming significantly lighter than previously utilized freight railcars, that is an empty or unloaded railcar weight less than 44000 pounds versus earlier freight railcars of 63000 pounds. This change in weight induces changes in railcar-tracking on superelevated curves, railcar instability, body rocking, truck squaring, and other problems associated with railcar usage at both an empty and fully laden state. These variations or instabilities in railcar operating characteristics are known to lead to operational problems, including derailments at an extreme condition. The problem for any railcar builder, and the suppliers of freight railcar truck assemblies, is to match the design of the truck assembly to the physical parameters of the freight railcar to enhance its safe operation.
One of the primary methods for dealing with the oscillations of a railcar and truck assembly is the damping from the above-noted friction shoe, as well as the stabilizing effect of the supporting springs. These oscillations may be due in part to the physical track conditions experienced by freight cars during their operation. Variations in track conditions can effect operation of the truck assembly, which track variation effects may be amplified as they are transferred through the wheel, axle and suspension to the frame. This may effect operation of the railcar as it traverses the track and encounters more of these track-induced operating problems.
The spring rates for the dynamic or moving car condition are graphically illustrable as generally having a range of spring deflections, that is expansions or compressions, centered about the static compressed height of the spring for both the empty-car weight and the laden-to-capacity railcar weight. The spring rate for each spring of a two-spring combination is linear over its individual range, but considered as a unit the springs may represent a variable rate spring. A variable rate spring arrangement is required to provide the broad spring-rate operating range necessary for railcar stability at a car-empty state for light weight railcars now contemplated for use by the railroads; and, also to provide the requisite suspension and damping to the fully-laden railcar at a greater weight than the present rated-standard capacity.